JPS6249152B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a laser processing machine using an optical fiber light guide, which improves the coupling characteristics of a carbon dioxide laser (hereinafter referred to as CO ₂ laser) output beam to the optical fiber. The purpose of this invention is to provide a laser processing machine with

The CO ₂ laser scalpel with an optical fiber light guide can be said to be a laser scalpel with high future potential, as it has high operability due to the improved flexibility of the light guide and can be used as an endoscope laser scalpel. In recent years, research results on infrared fibers that transmit CO ₂ laser light have been published, but among them, KRS-5 (TlI, TlBr mixed crystal) has the highest destruction threshold, and the beam emitted from the fiber is Intensity is reported to be 20W, and 100W for short-term use within a few seconds.

On the other hand, the drawback of KRS-5 fiber is its low transmission characteristics. Propagation loss can be roughly divided into two types: Fresnel reflection loss at the fiber entrance and exit end faces, and absorption and scattering loss when passing through the fiber.The former is particularly large due to its high refractive index (reflectance 16.5%/surface), and the overall The transmittance reaches a low value of about 60%, which significantly reduces the efficiency of laser energy use.

The basic configuration of the conventional fiber laser scalpel is explained first.
As shown in the figure. In the figure, 1 is a laser oscillator, 2 is a laser power supply, 3 is a laser beam, 4 is a fiber coupling lens, 5 is an optical fiber light guide path, 51
52 is an entrance end face of the fiber, 52 is an exit end face of the fiber, 6 is a condenser lens, 7 is a surgical object, and 8 is a focal point. The laser beam 3 from the laser oscillator 1 has a diameter of approximately 10 mmφ,
The divergence angle is 1 to 2 mrad, and the light is focused onto the fiber entrance end face 51 to a diameter of 0.3 to 0.4 mmφ or less by the coupling lens 4 and introduced into the fiber 5. As mentioned above, the reflection loss at the incident end face 51 is KRS-5.
The CO ₂ laser beam combination is 16.5%.

This reflection loss and the same reflection loss at the fiber exit end face 52 can be reduced by applying an antireflection film to both surfaces, and it is theoretically possible to increase the total transmittance to about 90%.
However, as mentioned above, the laser beam is focused on the fiber entrance end face 51 to a diameter of approximately 0.3 mmφ, so if the incident laser beam intensity is 50 W, the energy density on the antireflection film is approximately 70 KW/cm ² . Unfortunately, antireflection films with this level of light resistance are currently unavailable.

If the laser beam 3 is made into linearly polarized light and is made to enter at the fiber entrance end face 51 at the Brewster angle, reflection loss at the entrance end face 51 can be eliminated. However, the normal laser scalpel optical
Since the fiber 5 does not maintain the plane of polarization during propagation of light, the light is no longer linearly polarized at the exit end surface 52, and even if this surface is polished to the Brewster angle, Fresnel reflection loss still remains.

The present invention uses a laser processing machine consisting of a combination of an optical fiber and a laser to polish both the fiber entrance and exit end faces so that the condition of Brewster's angle incidence is satisfied, and to reduce Fresnel reflection loss on both the entrance end face and the exit end face. The objective is to provide a laser scalpel that prevents the damage caused by oxidation, increases overall transmittance to approximately 90%, and greatly improves laser utilization efficiency. Its basic configuration is shown in Figure 2.

In the figure, 11 is a linearly polarized laser oscillator;
Reference numeral 2 indicates a power source for the laser, and reference numeral 13 indicates a linearly polarized laser beam, which is focused onto the incident end surface 510 of the optical fiber 15 by the coupling lens 4 . The incident end face 510 is obliquely polished so that the incident angle is the Brewster angle, and there is no Fresnel reflection loss. Reference numeral 15 denotes a single polarized optical fiber, and its output end face 520 is also the input end face 5.
Similar to No. 10, Brilleuster angle polishing has been performed to eliminate Fresnel reflection loss. The light emitted from the fiber is condensed by a condenser lens 6 and irradiated onto a focal point 8 on a surgical object 7 .

Now, single polarization optical fiber 15
In recent years, visible and near-infrared optical fibers have been announced in the fields of communications and information processing. Although there are no products announced for far-infrared fibers for power transmission, the principle is the same, so they can be manufactured by modifying the structure of current fibers for CO ₂ lasers. The principle is orthogonal x, y on the fiber cross section.
The object of this invention is to set the axial propagation constants βx and βy of the polarized light waves Ex and Ey modes with respect to the coordinate axes to different values with a large difference, thereby preventing conversion between the two modes. Two methods have been proposed for this, the first is x,
The second method is to make use of the birefringence phenomenon that makes the refractive index different in the y direction.The second method is to make the cross section of the fiber a special shape that is not a perfect circle but has different extensions in the x and y directions. The cross-sectional shapes of two or three of the latter are shown in FIGS. 3, 4, and 5. FIG. 3 shows a circular shape, FIG. 4 shows a rectangular shape, and FIG. 5 shows a star shape. Since the propagation distances in the x and y directions are different in each case, βx and βy are separated. In terms of the inertia cross section in Figure 3, the extinction ratio _η = | _{e y} | ₂ (2) becomes about -28 dB. The far-infrared KRS-5 fiber for CO ₂ laser light is manufactured by heating extrusion from a die, so the optical fibers with the cross-sectional shapes shown in Figures 3 to 5 can be manufactured by changing the die shape to the desired cross-sectional area shape. It is found by making them equal.

As a single polarized optical fiber,
In addition to having a shape with different extensions in the x and y directions as described above, it is also possible to make the cross section of the fiber a perfect circle and make the refractive index different in the x and y directions. The present invention can also be realized with an optical fiber having such a configuration. In a perfect circular fiber, E _x and E _y mode light waves propagate while mutually converting each other, so even if a linearly polarized light wave is incident as shown in Figure 6a and b, the propagation constant difference △β = βx −βy(βx:
E _x mode axial propagation constant, βy: E _y mode axial propagation constant) (1) Linearly polarized light △βz=0 (3) ↓ (2) Circularly polarized light 0<△βz<π/2 (4 ) ↓ (3) Circularly polarized light △βz=π/2 (5) ↓ (4) Circularly polarized light π/2<△βz<π (6) ↓ (5) Linearly polarized light △βz=π (7) With the cycle The polarization state shows periodic fluctuations (see Figure 6a), and the linear polarization state when the light enters the incident surface is reproduced every time z=L, which corresponds to △βz=2mπ m=1, 2,... (8) . This L is called the beat length, and by measuring the scattered light seen from the coordinate axis of the polarization direction as shown in the scattered light intensity in FIG. 6b, it is possible to find z that satisfies equation (8). In the laser scalpel with the configuration of the present invention shown in Fig. 2, the optical fiber length can be found from equation (8): z = 2mπ/△β (9) However, when m = 1, 2,... Since the star angle condition is satisfied, Fresnel reflection loss can be prevented. In this case as well, a total transmittance of about 90% can be achieved.

As described above, according to the present invention, it is possible to prevent the high Fresnel reflection loss that inevitably occurs when coupling CO ₂ laser light to an optical fiber, and increase the overall transmittance of the optical fiber to approximately 90%. , it is possible to improve the efficiency of laser energy utilization in a laser scalpel or processing device that uses a CO ₂ laser beam in combination with an optical fiber light guide.

[Brief explanation of the drawing]

Fig. 1 is a conceptual diagram showing the basic configuration of a conventional optical fiber light guide type laser processing machine, and Fig. 2 is a single polarized light type optical fiber light guide laser processing machine according to the first embodiment of the present invention. Figures 3, 4, and 5 are conceptual diagrams showing the basic configuration of the machine; Figures 3, 4, and 5 are cross-sectional diagrams of single-polarized optical fibers; Figure 6a is a normal optical fiber that is not single-polarized. - A diagram showing how the polarization state changes due to transmission when a linearly polarized light wave is incident on the fiber, and b shows how the scattered light intensity changes at that time. 1...Laser resonator, 2...Common power supply, 3...Laser beam, 4...Coupling lens, 5...Optical
Fiber light guide path, 51... Entrance end face as vertically polished, 52... Output end face, 510... Input end face polished for Brewster angle incidence, 520... Output end face, 6... Condensing lens, 7 ...Object to be irradiated, 8...Focal point, 11...Linearly polarized laser, 13...Linearly polarized laser light, 15...Single polarized optical fiber.

Claims (1)

[Scope of Claims] 1. A laser oscillator that emits a linearly polarized light wave is configured to guide an output beam from a laser oscillator to an object to be irradiated via an optical fiber light guide, and the input and output end faces of the optical fiber light guide are 1. A laser processing machine, wherein the optical fiber light guide path has different propagation constants in two orthogonal directions on its cross section. 2. The laser processing machine according to claim 1, wherein the optical fiber light guide has different refractive indexes in two orthogonal directions on the cross section. 3. The laser processing machine according to claim 1, wherein the cross-sectional shape of the optical fiber light guide path is either a rectangle or a gourd shape. 4 When the difference in propagation constant between two orthogonal directions on the cross section of the optical fiber light guide is Δβ, the length Z of the optical fiber light guide is set as Z=2mπ/Δβ, m=1, 2... A laser processing machine according to claim 1.